The Mpv17 mutation was generated by infection of CFW preimplantation mouse embryos with a recombinant retrovirus, MPSVneo, which carries a neo gene and is a replication-defective derivative of the myeloproliferative sarcoma virus, a Moloney murine leukemia virus-related RNA virus. The insertion site. Sequencing data show that the provirus is situated in intron one of the gene in the transcriptional orientation opposite to that of the endogenous coding region.